1. Field of the Invention
This invention generally relates to a chain tensioning device. More specifically, the present invention relates to a chain tensioning device that is provided with a damper unit, which slows sudden movement of the chain guide in at least one direction.
2. Background Information
A bicycle typically uses a chain drive transmission for transmitting a pedaling force to a rear wheel. Many bicycles having a chain drive transmission either use an internally geared hub or a pair of derailleurs to selectively change speeds. In the case of an internally geared hub, a chain tensioning device (also called a chain tensioner) is used to keep the proper tension on the chain. In the case of a chain drive transmission using a pair of derailleurs, the rear derailleur is usually configured to provide the proper tension on the chain. Thus, the rear derailleur is a type of a chain tensioning device that is also used to shift the chain between rear sprockets. A typical rear derailleur has a base member, a movable member pivotally supporting a chain guide and a linkage assembly (e.g., a moving mechanism) coupled between the base member and the movable member so that the chain guide moves laterally relative to the base member. The rear derailleur typically has a biasing element that is provided between the movable member and the chain guide to bias the chain guide in a direction to take up slack in the chain.
In some cases, a chain tensioning device is provided with a damper to slow changes in chain tension. An example of a chain tensioning device that is provided with a damper is disclosed in International Patent Application Publication No. WO 2007/106897. The chain tensioning device disclosed in this international patent application publication is a rear derailleur in which the damper includes a one-way valve to slow changes in chain tension in one rotational direction of the chain guide more than the other rotational direction of the chain guide. Another example of a rear derailleur (i.e., a chain tensioning device) is disclosed in U.S. Pat. No. 6,135,904.